1. Field of the Invention
This invention relates to a keel arrangement for a sailboat hull, and particularly a keel arrangement for a high performance yacht, such as the kind of yacht used in races for the America's Cup.
2. Prior Art
Conventional yachts have a fore-and aft ballast member at the bottom of a central keel. Yachts have also been made in which a ballast member is suspended below the hull, at its front and rear ends, by twin foils (also referred to as "wings"), separated by a space. Sometimes the front foil is termed a "keel", while the rear foil, of which at least a part is pivotable about an upright axis, is termed a "rudder", and the whole arrangement may be termed a "tandem keel". Various designs of twin foil or tandem keel arrangements are shown in U.S. Pat. No. 5,313,905, which issued May 24,1994, to Calderon.
The foils used in these keel arrangements may be wholly pivotable about their upright axes, with the foils being connected by bearings to both the hull and the ballast body. Alternatively, a front part of a foil may be made as a fixed strut connecting the hull to the ballast body, and have a pivotable rear flap mounted on this front part. The first arrangement is more efficient hydrodynamically than the second, and the present invention is concerned with designs in which at least the rear foil or rudder (hereinafter referred to only as the "rear foil") is wholly pivotable. However, such designs have the disadvantage that the bearings holding the foils are subjected to considerable stress, since each is taking about one half of all the forces to which the ballast member is subjected. These forces include both hydrodynamic lift and drag forces on the foils and ballast member. Additionally, torque forces also occur in the ballast member caused by differential bending of the front and rear foil. These forces tend to strain the entire arrangement in such a way as to cause the bearings that attach the foils to the ballast member to jam. To date the twin foil yachts which have been built have experienced such large forces on the rear foil that it has not been possible for a helmsman controlling this to detect changing hydrodynamic loads due to the changing rear foil angles of attack used in the process of steering. These unusually large loads require additional mechanical advantage in the steering mechanism, and tend to smother hydrodynamic forces usually conveyed to the helmsman, and make the experience gained in the steering of a sail boat with a detached rudder of little use. Feedback of the hydrodynamic forces from the front foil has also been largely precluded in the known designs.